1. Field of the Invention
The present invention relates to a multi-quantum well (MQW) structure semiconductor laser element, and more particularly to a multi-quantum well structure semiconductor laser element into which an optical modulator is integrated.
2. Description of the Related Art
Recently, the studies on ultra-high speed and large capacity data communication and data processing systems utilizing optical techniques are rapidly advancing, and the availability of advanced growth techniques such as a metal-organic vapor phase epitaxy (MOVPE) has stimulated the research and development of various elements such as an ultra-high speed semiconductor photonic element whose operation speed is beyond 2 Gb/s, a quantum well structure element, a surface emitting type semiconductor laser, and a semiconductor photonic integrated circuit (SPIC).
The ultra-high speed semiconductor photonic elements include, as examples, a Distributed Feedback Laser Diode (DFB LD) and a Distributed Bragg Reflector Laser Diode (DBR LD). However, where the semiconductor laser is directly modulated, the spectrum width increases due to the changes in refractive indices in laser media that are caused by changes in injected carriers, and this results in the so-called "dynamic wave chirping". This wave chirping becomes a cause for limiting the transmission distance for high speed modulation, so that an external modulator which does not rely on the direct modulation has been proposed.
The examples of the modulators using semiconductor include a bulk type modulator in which a bulk semiconductor is employed as an absorption layer and which utilizes shifts of absorption edge due to Franz-Keldysh effects, and a quantum well structure modulator which utilizes shifts of absorption edge are larger than in the bulk type modulator due to Quantum-Confined Stark Effects (QCSE). Another example is a Mach-Zehnder type modulator which utilizes optical phase modulation on the principle of Mach-Zehnder interferometer.
Along with the fast and remarkable advancement in recent years in the crystal growth techniques, such as MOVPE (metal-organic vapor phase epitaxy) and MBE (molecular beam epitaxy), for growing thin films, it has become possible to form a high quality semiconductor hetero-junction interface having steep composition variations whose precision is accountable by a thickness of an atomic mono-layer. The potential well structure and the grating structure having the above-mentioned hetero-junction have special optical and electrical characteristics attributable to wave nature of electrons, and the application of these characteristics to the fabrication of devices is being vigorously pursued.
Recent publications demonstrating the control of compositions and thicknesses of semiconductor layers within semiconductor substrates are attracting considerable attention. For example, there is a report by O. Kayser in Journal of Crystal Growth, No. 107 (1991), pp. 989-998 on selective growth using an SiO.sub.2 film as a mask. Also, there is a report by K. Kato et al. in European Conference on Optical Communication of 1991, WeB7-1 on an optical modulator integrated type MQW DFB Laser Diode, in which the mechanism of selective growth as above is utilized. Also, Takano et al. have reported in ECOC' 1992, TuB5-3 on Wavelength-Tunable MQW--DBR--LDs, in which the selective growth mechanism as above is also utilized.
With the integrated type modulator as above, the advantages achieved are that the output is high because there is no interconnection loss unlike with a discrete modulator requiring connection by an optical fiber, and that the handling is easy and secured because no complex optical systems are used.
However, in the multi-quantum well type modulator as described above, the carriers produced by the absorption of light are blocked from flowing out due to potential barrier walls of the multi-quantum well and are piled up in the multi-quantum well. The problems which result therefrom include the deterioration of the optical quenching ratio due to the internal electric field produced by the piled-up carriers.
Also, in the Mach-Zehnder type modulator described above, since the phase modulation is utilized, the element is long so that, in the case of an integrated type, one of the problems is that the dimension of the element becomes large thereby placing limitations to modulation bandwidth.